public class Circle {
    private double r;

    public void setRadius(double radius) { this.r = radius; }
    public double getRadius() { return r; }
    public double mianji() { return Math.PI * r * r; }
    public double zhouchang() { return 2 * Math.PI * r; }

    public static void main(String[] args) {
        Circle circle = new Circle();
        circle.setRadius(6);

        System.out.println("半径: " + circle.getRadius());
        System.out.println("面积: " + circle.mianji());
        System.out.println("周长: " + circle.zhouchang());
    }
}
